Influence of Mg on the temperature‐dependent optical properties of CdTe quantum dots embedded in Zn _0.7 Mg _0.3 Te

Abstract

We compare the temperature-dependent optical properties of CdTe quantum dots with and without Mg in their Zn(Mg)Te barriers. The difficulty of forming CdTe dots on Zn 0.7 Mg 0.3 Te barriers (due to the decrease of the lattice mismatch) has been overcome by using a new technique based on an efficient reduction of the surface energy. Mg incorporation in the barriers leads to a better heavy-hole confinement along the growth axis, which is manifested in PL studies by both an extension of the radiative regime temperature range and a strong increase of the activation energy for the non-radiative mechanisms. However, the in-plane confinement is less enhanced, which leads to progressive inter-dot carrier transfers with increasing temperature, as evidenced directly by the analysis of photoluminescence intensities for different single dots. Our temperature-dependent data (time-resolved and micro-photoluminescence) indicate that this transfer consists of a thermally activated process via the two-dimensional wetting-layer states.1 Introduction In recent years much attention has been paid to the fabrication of semiconductor quantum dots (QDs) and to their specific optical properties. In particular, optical studies at low temperatures (typically below few tens of Kelvins) provide very interesting information on the basic physics of confined excitons. They also show the potential of quantum dots as active media for opto-electronic devices such as, for instance, single photon emitters. However, to go further, one has to demonstrate the feasibility of such devices at higher temperature. For this purpose, it is important (i) to use materials that have a large exciton binding energy, and (ii) to understand and control the mechanisms responsible for the thermal escape of excitons from 0D confined states towards non-radiative channels.The strong Coulomb interaction in II-VI QDs as compared to most III-Vs (except GaN dots) makes them very attractive for single photon generation as demonstrated recently at intermediate temperature with CdSe QDs [1]. For CdTe/ZnTe QDs however, emission at room temperature has not yet been observed, neither for an ensemble of QDs [2,3], nor for a spatially selected individual dot [4]. This can be attributed to the very small unstrained valence band offset (VBO) of the CdTe/ZnTe system [5,6].This limitation may be overcome however, by incorporating Mg into ZnTe barriers. Calculations have predicted a VBO of 63% for ZnTe/MgTe [6] (because of the lack of Mg d-orbitals) and VBOs as large as 30% and 87% have been found experimentally for telluride (CdTe/CdMgTe QW) [7] and selenide (ZnSe/ZnMgSe QW) [8] systems, respectively. Thus, incorporating Mg into ZnTe barriers is expected to increase the potential confinement for the heavy hole, and so to significantly strengthen the excitonic emission.In this paper, we compare the temperature-dependent optical properties of CdTe self-assembled quantum dots embedded in either pure ZnTe or Zn 0.7 Mg 0.3 Te barriers and we show the improvement brought